1. Field of the Invention
The present invention relates to a solenoid controlled valve for a and to a variable displacement compressor used for compressing refrigerant in a refrigeration cycle of an automobile air conditioner or the like.
Since the compressor of a refrigeration cycle of an automobile air conditioner is directly connected to the engine by a belt, the speed of the compressor cannot be controlled independently. So, conventionally a variable displacement compressor is used apt to change the amount of refrigerant (the amount of discharge) to obtain appropriate cooling capacity without being restricted by the driving speed of the engine. In the variable displacement compressor, a swing plate with a variable angle of inclination is installed in an airtight crank chamber. The swing plate is driven by rotational movement of an axis of rotation and executes a swing movement. A piston coupled to the swing plate executes reciprocal motions and is received within a cylinder so that it sucks refrigerant from a suction chamber connected to a low-pressure refrigerant pipe conduit into said cylinder, compresses it, and discharges it into a discharge chamber connected to a high-pressure refrigerant pipe conduit. The amount of discharge of the refrigerant is varied by changing the angle of inclination of the swing plate in accordance to a variation of the pressure in the crank chamber. The pressure in the crank chamber is controlled by a solenoid controlled valve. The variable displacement compressor continues to operate with a minimum displacement which is about 5% of the maximum displacement even when there is no cooling demand. That is, the compressor is then operating at its minimum operation state. However, if operated like that the problem occurs that fins of an evaporator supplied with refrigerant by the compressor are freezing when the load is small as in winter even at the minimum operation state, due to the flow of compressed refrigerant into the evaporator.
2. Description of the Related Art
It is known to install a valve in a suction passage extending between the low-pressure refrigerant pipe conduit and the suction chamber which valve closes at the minimum operation state, thereby preventing low-pressure refrigerant from being sucked into the compressor. In a conventional variable displacement compressor the swing plate is driving said valve for opening and closing the suction passage. When the swing plate is driving the valve for opening and closing the suction passage it is necessary to install a mechanical transmission mechanism between the swing plate in the crank chamber and the valve in the suction passage which is provided outside of the crank chamber. Said mechanical transmission mechanism needs proper sealing between the crank chamber and the valve so the transmission mechanism is complicated due to these problems.
Furthermore, instead a solenoid controlled valve can be provided which then is responsible for varying the pressure in the crank chamber to vary the amount of displacement. A known solenoid controlled valve for that purpose comprises a pressure sensitive part between the moveable iron core of the solenoid and the valve controlling the pressure in the crank chamber. The pressure sensitive part is actuated by the varying suction chamber pressure. When de-energising the solenoid the spring force of the spring for operation urging the valve in opening direction should suffice to maintain said valve in its open state and so that the pressure in the crank chamber will correspond to the pressure in the discharge chamber and the compressor should remain in the minimum operation state. However, since the pressure sensitive part is actively situated between the moveable iron core of the solenoid and the valve actuating rod a change of the suction chamber pressure may lead to a response of said pressure sensitive part resulting in an actuation of the valve into or towards its closed state. So, it is impossible to keep the compressor in the steady operation state of the minimum displacement. It is necessary to separately provide a clutch or the like in a driving part of the compressor resulting in an increase of the overall costs.